Polypropylene resins are relatively cheap and have good properties, and so have a widespread use such as various molded articles, e.g., injection molded articles such as sheets, containers, caps and those having a hinge structure; films; and blow molded articles such as air ducts. Films of polypropylene resins have been extensively employed as packaging materials for foods and fiber products, taking advantage of their mechanical, optical and thermal properties.
In compliance with the properties required for the molding applications, there have been used polypropylene compositions, each based on propylene homopolymer; random copolymers of propylene and .alpha.-olefins, especially ethylene; and propylene homopolymer/propylene-.alpha.-olefin copolymer.
In general, propylene homopolymer has high rigidity, good heat resistance, but insufficient flexibility, poor impact resistance, especially low-temperature impact resistance and poor tear strength when it is formed into films. Random copolymers of propylene and .alpha.-olefins have good, transparency and good flexibility, but poor heat resistance and poor low-temperature impact resistance. Since known propylene compositions comprising a propylene homopolymer and a propylene-ethylene copolymer have good heat resistance and low-temperature impact resistance, they have been extensively employed in a wide variety of industrial fields including automobiles and appliances. However, they are inferior in transparency and gloss to homopolymer, but have high mold shrinkage factor like homopolymer, and also they are inferior in tear strength and flexibility to copolymers. In any case, a phenomenon of whitening is observed in said known compositions when impacting and bending, and their whitening resistance is insufficient.
The methods to improve their insufficient properties include a method of modifying the properties of homopolymers, copolymers or compositions and a method using additives.
For example, a general method has been employed wherein the mold shrinkage factor of the composition is improved by filling with inorganic materials, such as talc. However, the method requires a large quantity of inorganic materials such as talc to improve the mold shrinkage factor, which results in an increased weight of the molded article and a remarkably poor appearance. Further, a method has been employed wherein an ethylene content in the copolymer of the composition is lowered to improve the transparency and gloss. However, it is general that lowering the ethylene content in the copolymer contributes to an improvement in gloss, with the reduction in low-temperature impact resistance and rigidity.
JP-A-60-28411 discloses a process of producing a high-rigidity ethylene-propylene copolymer, i.e., a propylene composition wherein propylene and ethylene are copolymerized in three consecutive stages with varying the ethylene content. The composition obtained by this process has high rigidity, good whitening resistance, good impact resistance and good heat resistance, but more improvements are required in the transparency, gloss, appearance of the molded articles and mold shrinkage factor on molding.
JP-B-7-30145 discloses a propylene block copolymer, i.e., a propylene composition which comprises a crystalline polypropylene block and an ethylene-propylene random copolymer block, the content of the crystalline polypropylene component being 55-95 wt. %, the ratio of the intrinsic viscosities of both components ([.eta.].sub.EP /[.eta.].sub.PP) being 0.5-2.0 and a glass transition temperature of the ethylene-propylene random copolymer being -30.degree. C. or lower. This composition has substantially the same range in respect of the content of the ethylene-propylene copolymer and the ratio of the intrinsic viscosity of the propylene homopolymer to that of the copolymer, has good mechanical properties such as impact resistance and rigidity, but more improvements are required in the transparency, gloss and appearance, especially stress-whitening resistance of the molded articles, and the mold shrinkage factor on molding.
JP-A-6-93061 discloses a melt-kneaded polypropylene block copolymer obtained by polymerizing a monomer based on propylene to form a polymer comprising polypropylene (component A) in an amount of 60-80% by weight to the total polymer amount, and then polymerizing a mixture of ethylene and propylene to form an ethylene-propylene copolymer (component B) having an ethylene content of 20-50% by weight, and melt-kneading the resulting polypropylene block copolymer, wherein the intrinsic viscosity ([.eta..sub.B ]) of the ethylene-propylene copolymer is at least 2.0 dl/g and the ratio ([.eta.].sub.B /[.eta.].sub.A) Of the intrinsic viscosity of the component B to the component A is 1.8 or less. This composition has substantially the same range in respect of the content of the ethylene-propylene copolymer and the ratio of the intrinsic viscosity of the propylene homopolymer to the copolymer, has good low-temperature impact resistance, good blocking resistance and good appearance, but more improvements are required in mechanical properties such as rigidity and mold shrinkage factor on molding.
JP-A-6-328640 discloses a multilayered film with a largely improved tear strength wherein a polypropylene resin layer is laminated on both surfaces of a straight chain low-density polyethylene layer. In the multilayered film, a compatibility of,the laminating low-density polyethylene with polypropylene resin may be not substantially good, and so fish eye is easy to occur when the film is remelted and reused.
JP-A-52-126450 discloses a blend of three components comprising an ethylene-.alpha.-olefin copolymer rubber, polypropylene or high-density polyethylene and paraffin or naphthene oil. This blend has good flexibility, but lower heat resistance.
JP-A-4-282232 discloses a resin tubular body wherein a rigid portion produced from a rigid polyolefin resin and a flexible portion produced from a thermoplastic elastomer are connected in series with each other. This tubular body may be insufficient in the bonding strength between the rigid portion and the flexible portion.